Hi Debs!
Boy, can I relate to your situation! I have to be at work by 8, though, so I normally workout in the morning. Since becomeing pregnant, however (I'm 31 weeks now), it hasn't been happening as often as I would like.
My first piece of advice to you is to don't be too hard on yourself. I think it is fairly normal to feel more tired and less motivated to work out when you're pregnant. The way I have been handling it is to not hold myself to the same set of standards I would have prior to becoming pregnant. Now, I shoot for 3-4 workouts a week, whereas before I would aim for 5-6. I still try to do my early morning workouts because they work best for me and my work situation, but I compromise by only getting up early 3 times a week (Mon, Wed, and Fri) and then saving my 4th workout for the weekend. This helps me feel less tired and it's kind of a mental trick--when the alarm goes off, I tell myself to just get up and do it, and tomorrow I don't have to because it's my day to "sleep in".

Maybe something similar could work for you, only in the afternoons? Just come home from work, change clothes and head straight to your workout area, all the while reminding yourself that tomorrow you get to rest after work instead.
I have also shortened my workouts--before I would go for an hour minimum, now I only schedule 45 minutes, tops. So I am not setting my alarm quite as early as I did pre-pregnancy. For you, it would mean knowing you'll be done with your workout sooner after work than you were pre-pregnancy and you will therefore be able to rest sooner afterwards.
ABove all, I would say that if you are REALLY feeling tired on a particular day, don't push yourself. This is a time to take it easy on yourself. There have been mornings (like today!! oops!) that no matter what little head games I play or how much sleep I got, I still can't bear the thought of dragging myself out of bed to workout. I take it as a sign that I need a little extra rest that day, and move on. No biggie--your body is working hard to make that little baby, so it's not inconceivable that you're going to need extra rest/sleep whatever to recuperate some days!
Good luck on your pregnancy, and on keeping up those workouts!
Renae